The invention relates to treating materials so that hazardous and toxic materials contained in them are either destroyed or made safe.
Landfills have in the past been collection sites for a wide variety of discarded objects and materials. Some contain substantial levels of toxic and hazardous chemicals whose clean-up is being required by government regulation. Of concern are, for example, materials containing heavy metals such as lead, nickel and chromium or toxic halogen containing chemicals such as polychlorinated biphenyls (PCB's).
For convenience, excavated landfill material will be referred to as ELM.
Incineration processes have been used or proposed to treat the ELM by burning it along with other combustible material, typically municipal garbage. Such incineration processes are feasible but may involve large flow rates of potentially harmful materials and the incinerator ash that results as an end product may not be environmentally benign; potential for leaching of toxic heavy metals may still exist.
One type of system for treating ELM is a pyrolyzer such as that disclosed in copending application Ser. No. 027,775, filed Mar. 18, 1987 by Levin and assigned to the present assignee. Such a pyrolyzer produces vitreous, i.e., glass like, material by electrical heating in a chamber operated in a substantially closed or pyrolytic manner. Such a system incurs high energy costs which it is desirable to minimize It is however successful in producing vitrified material in which the heavy metals can be trapped in the glass matrix of the slag.
A plasma fired cupola is a known apparatus previously disclosed for such purposes as metal recovery as in Fey et al. U.S. Pat. No. 4,530,101, July 16, 1985; Dighe et al., "Plasma Fired Cupola and Innovation In Iron Foundry Melting", AFS transaction paper, 1986; Dighe et al. U.S. Pat. No. 4,761,793, Aug. 2, 1988; U.S. Pat. No. 4,780,132, Oct. 25, 1988; and U.S. Pat. No. 4,769,065, Sept. 6, 1988; and copending applications Ser. No. 047,808, filed May 8, 1987, now U.S. Pat. No. 4,828,607 issued May 9, 1989, and Ser. No. 212,851, filed June 29, 1988, now U.S. Pat. No. 4,853,033 issued Aug. 1, 1989 by Dighe et al. and Ser. No. 226,712, filed Aug. 1, 1988, now U.S. Pat. No. 4,889,556 issued Dec. 26, 1989 by Dighe, all assigned to the present assignee. The foregoing descriptions are incorporated herein by reference for general information on the structure and operation of plasma fired cupolas.
Cupolas, not plasma-fired, are presently known for metal and mineral melting that utilize a shaft with coke and blown air, sometimes enriched with oxygen, through tuyeres near the bottom. These units require such amounts of air that fine particles of charged material and even, in some instances, vitrified material, may be blown upward. Additionally, they normally achieve maximum temperatures of only about 3000.degree. F.
Among the purposes of the present invention are to provide an effective and economical treatment for excavated landfill material (ELM) containing toxic and hazardous materials. In the process of the present invention, a plasma fired cupola is used for treatment of ELM. The cupola is a vertical shaft with a charge door proximate the top thereof. A plasma torch is provided and located in a tuyere proximate the bottom of the cupola and the plasma torch has a feed nozzle. The plasma torch is electrically energized and produces a plasma from air. Air is fed through the nozzle and is heated to a high temperature by the plasma torch and supplied into the cupola.
In starting operation of the system, the cupola is partly filled with a carbonaceous fuel, such as coke or a mixture of coke and coal, which is ignited. When adequate operating temperature is reached, the charge material is feed through the charge door. The ELM is normally supplied along with reactive agents such as additional coke and a fluxing agent, such as limestone.
Conditions are maintained for melting the ELM in the cupola to form a vitreous slag. In addition, metal supplied with the ELM, such as iron and copper, becomes molten and will separate from the slag gravimetrically. The coke reduces oxides of such metals to yield the metal itself. The cupola is tapped to take out the vitreous slag and the molten metal.
The off gases from the cupola are allowed to rise to an afterburner located above the cupola for destruction of any toxic and hazardous materials contained in the off gases. The vitreous slag from the cupola is allowed to cool and produce non-hazardous solid material from which heavy metals such as chromium, lead and nickel, occurring as oxides, are substantially non-leachable.
In accordance with the present invention, preferably about 6% of a charge is fuel. The fuel is sufficient to supply all of the combustion gases for the afterburner in which PCB's or the like are burned.
A layering technique is employed for the material supplied into the cupola through the charge door. That is, a distinct layer of fuel is provided between layers of limestone, ELM or mixtures thereof. The layering helps reduce back pressure in the system. The gas flow rate is reduced to a lower level than that employed for foundry melting, such as preferably about 0.6 cubic feet per minute per square inch of cupola cross-section as compared to about 0.9 cubic feet per minute. Further, the amount of limestone supplied is adjustable for optimizing the basicity of the slag to achieve a desired flow rate.
These and other aspects of the invention will become better understood from the description hereinafter